1. Field of the Invention
The present invention generally relates to loudspeakers and loudspeaker suspension systems, and more particularly, to a loudspeaker and suspension system that separates the lower suspension from the diaphragm and voice coil with a concentric tube member to provide a restoring force both above and below the moving assembly's center of rotation.
2. Description of the Prior Art
As high-excursion loudspeakers are more commonly used, weaknesses in traditional loudspeaker suspension are becoming more apparent. When large amounts of cone movement are required, lateral stability of the moving components (that is, the coil and former) becomes critically important. This is because clearances between the voice coil and magnetic gap boundaries are minimal, as seen in the prior art speaker of FIG. 1. Consequently, any deviation from the desired linear motion (i.e., rocking) can cause the voice coil to rub against the top plate and/or pole piece.
This rubbing can produce unacceptable levels of audible noise and may even lead to driver failure if the instability is severe.
In the typical loudspeaker, one cause of the moving component instability is the small physical distance between the suspension components, leaving little tolerance for rocking. The conventional suspension system comprises a roll 3 (outer suspension) and spider 2 (inner suspension). The roll 3 and spider 2 are the members responsible for controlling and stabilizing the position of the cone and voice coil relative to the motor structure. When the moving assembly begins to rotate away from its ideal linear path during its travel, the roll and spider are called on to resist and prevent rocking. The roll and spider, however, only have a limited amount of restoring force available to resist the rocking motion. This limited resistance to rocking is exacerbated by their close physical proximity to the center of rotation. Being located near the rotational center does not afford any leverage for augmenting the meager restoring force of the spider. In addition, the rotation center is volatile, whereby changes in excursion shift the rotation center, altering moment forces on the roll and spider. Notwithstanding, the spider remains relatively close to the center of rotation, such that any mechanical advantage in the typical suspension system is minimized by its proximal relationship to the axis of rotation.
High excursion loudspeakers further amplify problems with rocking, as high-excursion loudspeakers require relatively long voice coil winding lengths. A very significant portion of the moving assembly's weight consists of the copper wire in the voice coil. Longer winding lengths dictate that the copper wire in the coil be distributed over a greater linear distance along the voice coil former's axis. Consequently, more of the coil's weight is placed further away from the spider, affecting the position of the rotation center. This increases the pendulum-like behavior of the system and makes rocking modes harder to control.
High-excursion loudspeakers often compensate for rocking mode problems by increasing the width of the magnetic gap in which the voice coil resides. A larger clearance between the coil and the motor structure allows the speaker to tolerate more movement of the voice coil without contact. However, there is a downside to increasing the width of the magnetic gap. A larger magnetic gap decreases the magnetic flux density in the gap, and hence the driver's motor strength. Decreased power handling capability is another trade-off when enlarging the gap. This is because the metal parts are now farther away from the voice coil, making them less effective in transferring heat away from the coil body. Consequently, heat builds up in the voice coil, causing adhesives to soften, and creating more power-related failures.
Conventional high-excursion loudspeakers also require excessive loudspeaker mounting depth. The important clearance dimensions in a speaker design include the clearance between the neck joint (intersection of cone, spider, and voice coil former) and top plate, and between the bottom of the voice coil and the back plate. These clearances must be greater than or equal to the driver's maximum physical excursion capability. The speakers with a large cone excursion require a larger corresponding clearance within the frame structure to prevent moving parts from making contact during peaks. As the excursion capability increases, so does the depth of the loudspeaker structure. This problem can be troublesome in certain markets, such as car audio, since consumers typically desire the placement of very high performance drivers in extremely tight places.
Finally, it is also desirous to provide alternative ways to vent air pressure from under the diaphragm. An air volume defined by the traditional diaphragm, dust cap, voice coil inner diameter, and pole piece changes size as the loudspeaker's moving structure travels through its excursion range. If this pressure is not released, the cone's motion will be impeded, changing the woofer's performance specifications dynamically depending on excursion. This air pressure is usually vented through a vent machined in the pole piece. However, machining the vent increases the cost of the motor structure and can increase magnetic circuit losses due to the removal of metal from the pole. the instant invention offers several alternate means of venting this air pressure.
Several speaker designs may be available in the background art for increasing stability. However, none are known to solve the above-noted problems. In fact, there is no design known which is directly comparable to the concentric tube concept. Past attempts to increase the mechanical stability of loudspeakers have generally utilized dual spiders separated by some predetermined distance. However, any advantage realized in these designs is limited and offset by the need for an additional spider with little separation from the first spider. As the above-noted devices neither solves nor adequately addresses the problems contemplated by the present invention, there remains a need for a loudspeaker with increased stability and resistance to rocking.